Klausmobile Tube Tester Files

The Last DHT

Or the first DHT that went through Klaustester torture bench. Actually, it is a DH Pentode, but thanks to the late Aleks Belkanov it is labelled 'the last DHT' - it is a locktal tube from the fifties. Here is a triode-connected chart from Belkanov's webzine (shaded area is the tester's range limited by -10V grid source)

My tester is not fit for direct heating and pentodes, so I powered the tube from a separate battery with a series resistor. Battery negative applied to cathode center tap (pin 8), battery positive to cathode extremes (pin 1, 7). That is, the average cathode potential is 1V above ground (in case of true AC heating it will be exactly at ground). For pentode mode, I attached a separate 80V regulated PS (didn't have a higher voltage) to cathode center pin . Both add-ons don't induce any ground loops.

Test sample was quite close to specs.

What is unusual is very nice low-power performance. Curves are nice, although my experience is not as rosy: i tried it with 60..80V on plate, battery bias, and the low-current versions (3..6mA) were thin-sounding. Icreasing Ia to 15mA (Vg=-3V) added 'body', I suspect the tube likes generous currents. See my Projects: Baby of Camel page for schematics

Pentode mode measurements for 80V at G2, 0V at G3

Note the obvious kinked pattern at low voltage (Vp stepped by 2.5V)

At low Vp and positive Vg voltage gain, indeed, becomes negative (red curves on the chart above drop below the white curves).

Legend: White curves: Ig<5uA Yellow: 5<Ig<20uA Red: 20<Ig<300uA. Measurements terminate at Ig>300uA

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